The present invention relates to a fluid pressure reducing device having a spiral groove through which a fluid such as gas or liquid flows to reduce its pressure. The fluid pressure reducing device of the invention is suitable for use particularly as a refrigerant pressure reducing device for reducing the pressure of the liquid refrigerant compressed by a compressor in a refrigerant circuit of a refrigerator or an air conditioner.
The fluid pressure reducing device used hitherto has a form of a thin-walled copper tube of small diameter generally referred to as "capillary tube" wound in a loop-like form for an easier mounting in the refrigerator or the like apparatus. In producing this conventional fluid pressure reducing device, it is necessary to loop the tube at a large radius of curvature, for otherwise the cross-section of the tube may be distorted to adversely affect the pressure reducing performance. Consequently, the pressure reducing device occupies an uneconomically large space. In addition, the looped tube is quite unstable and has a small resistance to any external force, so that the tube has to be handled with great care to avoid any distortion or breakage.
In order to eliminate these disadvantages of the looped tube type pressure reducing device, in recent years, a fluid pressure reducing device has been proposed, with a cylindrical core having a peripheral spiral groove being fitted or screwed in a pipe. In this type of fluid pressure reducing device, the pressure of the fluid is reduced as the latter flows through a spiral passage defined between the peripheral spiral groove of the core and the inner surface of the pipe. This type of fluid pressure reducing device has a compact construction and exhibits a considerably high resistance to the external damaging force.
This type of fluid pressure reducing device, however, has the following disadvantage. Namely, for a smooth and tight fit of the core into the pipe, it is essential that the outer peripheral surface of the core and the inner peripheral surface of the pipe have to be finished at a high dimensional precision, so that the cost of production is uneconomical. In the case where the core is pressed and fitted into the pipe, it is extremely difficult to finely adjust the flow resistance in the pressure reducing device because, in this case, the fine adjustment of the size of the spiral passage can hardly be effected. Thus, in this case, it is difficult to produce a device having a desired flow resistance and this results in a low production yield. In contrast, in the case where the core is screwed into the pipe, it is comparatively easy to make a fine adjustment of the flow resistance, because the depth or length of screwing of the core into the pipe can be varied and adjusted at a high precision to permit a fine adjustment of the size of the spiral passage. In this case, however, it is necessary to use pipes having comparatively large wall thicknesses, because of the necessity for forming a mating screw thread in the inner peripheral surface of the pipe. The formation of the screw thread in the pipe increases the number of steps of the production process thereby resulting in an increase in the production cost.
It is possible to drive the core having a spiral groove directly into the wall of the pipe having no screw thread. This method, however, cannot be applied to thin-walled pipes because of a large possibility of rupture or cracking in the pipe. In addition, there are problems concerning the disposal of the metal chips produced as a result of the driving of the core. It is also to be pointed out that the driving of the core requires a much labor and time.
Thus, the known fluid pressure reducing device composed of a core with a spiral groove and a pipe receiving the core cannot be produced at a moderate cost because of the necessity for the high dimensional precision of the core and pipe, increased number of steps of production due to the fitting or screwing of the core into the pipe and so forth. In addition, the fine adjustment of the flow resistance is difficult to perform particularly in the case where the core is merely inserted and fitted in the pipe.